The present invention relates to a connecting material and a semiconductor device.
Adverse effects of the lead against human nerves and hematogenic systems have been proved. In Europe, the ELV Directive (End-of Life Vehicles Directive) restricting the use of a lead in vehicles and the RoHs (Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) prohibiting the use of a lead in electrical and electronic equipment have been put in force in October, 2000 and in July, 2006 respectively. Conventionally, solders used for electric connections of components of electrical and electronic equipment contained the lead. Depending on the melting point, solders are classified into three types including a high temperature solder, a medium temperature solder, and a low temperature solder. Medium temperature solders such as Sn—Ag—Cu based solder, Sn—Cu based solder, and the like, and low temperature solders such as Sn—Bi based solder, Sn—In based solder, and the like have been developed, put into practical use and met requirements of the ELV Directive and the RoHS Directive. In the meantime, lead content of such solders is high at 85% or more, and a lead-free highly heat resistant connecting material which would replace with the high lead solders having a high melting point has not yet been developed. For this reason, those high lead solders are exempted from application of the ELV Directive and the RoHS Directive mentioned above. However, the high lead solders contain 85 wt % or more of lead as their component, and, therefore, give a big environmental impact compared with the Sn—Pb eutectic solder prohibited by the RoHS Directive. Accordingly, development of an alternative connecting material in place of the high lead solders is desired anxiously.
Patent Literature 1 (Japanese Patent No. 3850135) discloses “a high temperature soldering Zn alloy comprising Al of 1 to 9 mass %, Ge of 0.05 to 1 mass %, and the rest including Zn and unavoidable impurities”.
Further, Patent Literature 2 (Japanese Patent No. 3945915) discloses “a soldering Zn alloy comprising Al of 1 to 9 mass %, Mg of more than 0.05 mass % and less than 0.5 mass %, Ga of 0.1 to 8 mass %, and the rest including Zn and unavoidable impurities”.
Further, Patent Literature 3 (Japanese Patent Application Laid-Open Publication No. 2008-126272) discloses “a connecting material comprising an Al-based alloy layer and Zn-based alloy layers respectively provided on each of outermost surfaces of the Al based alloy layer, and more particularly, a connecting material comprising an Al-based alloy layer having an Al content of 99 to 100 wt %, or a Zn-based alloy layer having a Zn content of 90 to 100 wt %”.
Here, an application example of a conventional highly heat resistant connecting material is described with reference to FIG. 1 and FIG. 2. FIG. 1 shows an example of a structure of a conventional semiconductor device, and FIG. 2 illustrates a flash which occurred due to re-melting of a solder in the conventional semiconductor device.
As shown in FIG. 1, semiconductor device 7 is fabricated by a method including the steps of: connecting (die bonding) a semiconductor element 1 onto a frame 2 with solder 3, wire bonding inner lead of a lead 5 and an electrode of the semiconductor element 1 to each other via a wire 4, and then sealing with a sealing resin 6 or inert gas.
The semiconductor device 7 is reflow-soldered onto a printed substrate with an Sn—Ag—Cu based medium temperature lead-free solder. A melting point of the Sn—Ag—Cu based lead-free solder is high at about 220° C. Therefore, it is assumed that the connecting part is heated up to 260° C. when reflow-soldered. For this reason, a high lead solder having a melting point higher than 290° C. is used to prevent re-melting of the connecting (die bonding) part when reflow-soldered.
Currently, developed medium temperature lead-free solders such as an Sn—Ag—Cu based solder or the like have a melting point of about 220° C. When used for die bonding a semiconductor element, the solder is melted when a semiconductor device is reflow-soldered onto the printed substrate. If circumference of the connecting part is molded with resin, melting of inner solder may cause the solder 3 to leak through an interface between the sealing resin 6 and the frame 2 due to volume expansion occurring during melting, as shown in FIG. 2, a so-called phenomenon of the flash. Yet, even if no leakage occurs, the solder acts to leak out. As a result, a big void 8 is formed in the solidified solder, which might cause a semiconductor device to be defective. As a candidate for an alternative material, Au-based solders such as an Au—Sn based alloy, an Au—Si based alloy, an Au—Ga based alloy, or the like, Zn based and Zn—Al based solders, and Bi based, Bi—Cu based, Bi—Ag based solders or the like have been reported and are being reviewed in the world in terms of the melting point.
However, an Au-based solder lacks versatility in terms of the cost as it contains Au of more than 80 wt % as a component, and it is a hard and brittle solder, as well. A Bi-based solder has a thermal conductivity of about 9 W/m.K, which is lower than that of existing high lead solders. Therefore, it may be assumed that it is difficult to apply such solder to a power semiconductor device, a power module, or the like which require high heat dissipation. Further, the solder is hard and brittle, as well. A Zn-based solder and a Zn—Al based solder have a high thermal conductivity of about 100 W/m.K, but such solders (particularly, Zn—Al based solder) are unlikely to wet. Further, those solders are hard and have a big coefficient of thermal expansion. Therefore, there is a problem that a semiconductor element is likely to be broken by thermal stress applied when cooling down after connecting. Further, since pure Zn is highly reactive, interface reactions proceed significantly when temperature becomes high. Therefore, even if a good connection could have been attained, high heat resistance cannot be obtained.
Further, as a connecting material which addresses problems with the Zn—Al based solder of being unlikely to wet and being hard, a method using a Zn/Al/Zn cladding material is disclosed. According to the disclosure, a superficial Zn layer ensures the wettability (connectivity), and an inner layer comprising Al of a soft metal has a stress relaxation performance sufficient to ensure the connection reliability. Further, melting points of Zn and Al are 420° C. and 660° C. respectively, and a melting point of a Zn—Al eutectic (Zn-6Al) generated by the diffusion of Zn and Al is 382° C. Thus, the connecting material has a high melting point and high heat resistance.